monkeymonk :
I am having the same problem I think. How much should I up the voltage?
You'll want to hold as many variables possible out of your equation. So start by ensuring the maximum for your memory clock, gpu clock, and shader clock. Ensure your Gpu hold a 2:1 Ratio while overclocking. Once you've hit the limit in which they fail; gradually move the voltage up one.
Then push your Gpu / Shader up. If it's successful, push your memory clock. Then repeat until it can't essentially fails.
The odd thing; you shouldn't need to boost your voltage at all to achieve 1.2 Ghz on your 680. You can attempt to try these settings:
Memory Clock: Add 400 Mhz;
Core Clock: Add 125 Mhz;
Core Volts: Add 100 mV;
Power Limit: 159%
I'm going off settings from my friends Asus; if you want to push it past that. I'd suggest modding the pcb board to attain higher voltage to push past 1.3 Ghz. But that is more advanced and if you can't keep the temperature low enough may be extremely dangerous to damage your card.
The settings above should push the card to around 1.253 Ghz; that is a relatively easy clock to obtain with this card. You can obviously start low and work your way up. But keep in mind; you'll still want the latest driver and ensure a nice power-supply.
Hope that helps.